With the increase of the broadening and strength of the application spectrum of electromagnetic waves, requirements for electromagnetic shielding optical windows in the field of aerospace equipment, advanced optical instrument, communication equipment, medical diagnostic equipment and confidential facilities, etc. are higher and higher. The optical window is mainly requested to have super strong capability of wide-band electromagnetic shielding, high light transmittance, and the less impact on optical imaging, observation and detection the better. For example, in the field of aerospace equipment, optical window of the aircraft must achieve a high-quality electromagnetic signal isolation inside and outside the cabin, which on the one hand can shield external electromagnetic interference and harmful electromagnetic signals to avoid failure of the electronic equipment in the cabin, and on the other hand prevent electromagnetic signals from transmitting out of the optical window and causing electromagnetic leakage during operation of the electronic equipment in the cabin. However, light transmission is an essential function of the optical window, so electromagnetic shielding of the optical window should reduce the impact on the transparency of the optical window as much as possible, in particular the impact on functions of optical detection or optical imaging. Similarly, optical window of advanced optical instrument should have as high light transmittance as possible and as low impact on image quality as possible, so as to achieve high-quality detection and measurement, and to prevent impact of electromagnetic interference on photodetectors inside the instrument; for confidential building facilities for government agencies, military command areas and important scientific research units, the window glass of the houses needs to be guaranteed in lighting and also designed with electromagnetic shielding to prevent secret from divulging due to important information transmitting out of the window in the form of electromagnetic radiation when indoor computers and other electronic equipment are at work; optical window of the medical electromagnetic isolation chamber needs to ensure that the vast majority of the indoor electromagnetic waves is shielded to prevent health damage to the outdoor operator for always suffering electromagnetic radiation, etc. Currently, a transparent conductive film, a metal induced transmission type multi-layer film structure, the band-block frequency selective surface and a metal grid having a millimeter or submillimeter period, etc. are mainly used for electromagnetic shielding of this type of optical window.
The transparent conductive film is a transparent metal oxide film comprising indium tin oxide as the main material, and is often used in occasions where the visible light wave band is transparent, but is not suitable for the wide lucent wave-band, because it does not have strong shielding capability though having wide microwave shielding wave-band. Metal-induced transmission type multilayer film structure realizes shielding of the electromagnetic waves by using a composite structure of multilayer metal thin films and dielectric films. It has relatively strong ability of shielding low-frequency microwaves, but its light transmission rate is not high because the transmission regions are mainly visible light and ultraviolet light. Frequency selective surface uses the periodic resonating unit structure to achieve the functions of a band-pass or band-block filter, and because of its high metal coverage, it can effectively reflect the interfering electromagnetic waves beyond the operating band, but the optical transmittance is relatively low thereby reducing the imaging quality for optical detection and causing difficulty in optical image processing, pattern recognition, target acquisition and tracking. In sum, all the above technical solutions are obviously deficient in meeting the two requirements of the optical window having high light transmittance at wide waveband and high electromagnetic shielding ability at wide frequency band. In contrast, a metal grid having a period in millimeter or sub-millimeter can achieve strong electromagnetic shielding at low frequency and wide waveband because its period is much shorter than the interfering electromagnetic wavelength. The period of the metal grid is much longer than the optical wavelength, and thus can guarantee transmittance of optical waveband. Therefore, the metal grid with a period in millimeter or sub-millimeter has good transparent and conductive performances and can meet the requirements of the optical window for high light transmittance and wideband electromagnetic shielding, so it has been widely applied in the technical field of electromagnetic shielding of an optical window.
1. In Patent No. 03115313.5 with the title of “An Electromagnetic Shielding Observation Window”, a single- or multiple-layer metal mesh and a semiconductor-like quantum well structure are used to form an electromagnetic shielding structure, which can achieve an electromagnetic shielding efficiency of over 50 dB within 10 GHz and a light transmittance of up to 50% or more at the high visible light transmission area.
2. In Patent No. 93242068.0 with the title of “Electromagnetic Shielding Glass”, an electromagnetic shielding structure is formed by sandwiching a conductive metal mesh between two layers of glass and adhering on the metal frame by using a conductive transparent film at the outer sides of the glass, and such structure has certain lighting property.
3. In Patent No. 94231862.5 with the title of “An Electromagnetic Shielding Observation Window Having no Moiré Fringe”, two layers of metal meshes which are different in number are placed in parallel, and their warp or weft form an included angle, so as to eliminate the Moiré fringe phenomenon and achieve a clearer view.
4. In Patent No. 02157954.7 with the title of “Highly Effective Information Leakage-Preventing Glass”, each side of the metal mesh is provided with a layer of polycarbonate film whose outer sides are attached to a glass layer, and heat pressing is performed to form an electromagnetic shielding structure; such structure has high shielding efficiency when the light transmittance comes up to 60%.
5. Patent No. 200610084149.8 with the title of “Electromagnetic Wave Shielding Film and Method for Manufacturing the Same” describes a highly transparent electromagnetic shielding film having a metal mesh pattern formed by the photolithography process, and the main object of the invention is to reduce consumption of the metal and overcome the environmental pollution resulting from the use of a curing adhesive between the metal layer and the film substrate.
6. The U.S. Pat. No. 4,871,220 with the title of “Short wavelength pass filter having a metal mesh on a semiconducting substrate” describes a metal mesh having a square-shaped structure, which is used for achieving the anti-electromagnetic interference performance of the optical window.
7. Patent No. 201010239355.8 with the title of “An Electromagnetic Shielding Conformal Optical Window Having a Weft-Warp Grid Structure” describes an electromagnetic shielding conformal optical window having a weft-warp metal grid structure formed by using the metal grid technology and the conformal optical window technology, and mainly solves the problem of the structure design of the metal grid of the conformal optical window and improves the electromagnetic shielding performance of the conformal optical window.
8. Patent No. 200610010066.4 with the title of “Electromagnetic Shielding Optical Window Having Ring Metal Grid Structure” describes a metal grid unit having a ring-shaped profile, which is used for achieving the electromagnetic shielding function of the optical window; as compared to the single-layer squared metal grid, the light transmittance and shielding capability have been improved, and stray light caused by high order diffraction has also been homogenized in a certain degree.
9. Patent No. 200810063988.0 with the title of “An electromagnetic shielding optical window having a double-layer squared metal grid structure” describes an electromagnetic shielding optical window which is formed by placing squared metal grids or metal meshes having the same structural parameters in parallel on both sides of an optical window or a transparent substrate, and the electromagnetic shielding efficiency is greatly improved without reducing the light transmittance.
10. Patent No. 200810063987.6 with the title of “An electromagnetic shielding optical window having a double-layer ring-shaped metal grid structure” describes an electromagnetic shielding optical window formed by loading two layers of ring metal grids to both sides of the optical window, and solves the problem that the high light transmittance and high electromagnetic shielding efficiency cannot be achieved simultaneously.
11. Jennifer I. Halman etc. from the United States Battelle Institute developed an inductive metal grid having a ring unit-based hub-spoke stripe structure and a multi-ring overlapping structure (Jennifer I. Halman, etc., “Predicted and measured transmission and diffraction by a metallic mesh coating”, Proc. SPIE, 2009, 7302: 73020Y-1-73020Y-8), and believed that by the function of the rings, such structure can achieve homogenization of the grid high-order diffraction distribution and low sidelobe, and is favorable for imaging.
12. Ian B. Murray from the US Exotic Electro-Optics Company, together with Victor Densmore and Vaibhav Bora from University of Arizona. US, etc. reported the impact on the diffraction property after introducing parameters into an inductive metal grid having a hub-spoke stripe structure and a multi-ring overlapping structure and designing with random distribution (Ian B. Murray, Victor Densmore, Vaibhav Bora et al., “Numerical comparison of grid pattern diffraction effects through measurement and modeling with OptiScan software”, Proc. SPIE, 2011, 8016: 80160U-1-80160U-15), and pointed out that spacing and diameter of the rings are set as random values within a certain range, which is helpful to improve the uniformity of the high-order diffraction distribution.
With the metal grid (or metal mesh) as the core device for shielding, the above solutions can achieve good electromagnetic shielding effect and certain light transmittance. However, when metal grid (or metal mesh) is used as the electromagnetic shielding structure, the impact of the optical waveband diffraction by the grid is inevitable. Since the period of the metal grid is in the magnitude of millimeter or sub-millimeter, in order to achieve high light transmittance, its metal line width is generally in the magnitude of micron and submicron, such configuration parameter has very intense diffraction effect in the optical waveband. Most of the energy of the incident light can be transmitted through the metal grid, and the transmitted portion comprises a zero-order diffracted light and high-order diffracted light. Generally, zero-order diffracted light is useful information for imaging and observation, and the high-order diffracted light constitutes stray light which interferes imaging and detection. Therefore, the proportion of the zero-order diffracted light should be increased as much as possible, and the high-order diffracted light should be allowed to be uniformly distributed if its occurrence cannot be avoided, so that the stray light thus formed becomes comparatively uniform background or noise.
Currently, the metal grids are mainly in the traditional squared grid structure, as the structure mainly adopted in Patent 1-6 (the structure in Patent 7 is a grid-like structure, for it is processed on a curved surface). Squared grid structure is inherently contradictory between light transmission and shielding capability, and cannot have both high light transmittance and high electromagnetic shielding efficiency. In particular, the high-order diffraction energy of the squared grid is mainly concentrated on two axes perpendicular to each other, causing certain impact on the imaging quality or even difficulty in application in occasions having high requirements for imaging quality. A change in the diffraction property of the grid usually requires a change in its structure feature. The above-mentioned patent 200610010066.4 with the title of “an electromagnetic shielding optical window having a ring metal grid structure” proposed to construct a ring metal grid by using metal rings, which overcome the defect of the concentrated distribution of the high-order diffraction energy of the squared metal grid and may ease the contradiction between its light transmission ability and shielding capability. In the above documents 11 and 12, Jennifer I. Halman et al. and Ian B. Murray et al. also proposed a ring unit-based metal grid structure for improving the uniformity of the high-order diffraction distribution. However, studies of Jennifer I. Halman etc. relate to a single period ring arrangement structure, and the direction of arrangement is determined, and its effect on regulating the high-order diffraction is comparative to that of the structure proposed in Patent No. 200610010066.4. Although Ian B. Murray et al. made further research and proposed a randomly overlapping ring structure, in which the diameter and spacing of the rings are set as values based on random distribution in a certain range and which achieves further improvement on the uniformity of the high-order diffraction distribution, such random distribution of the diameters and spaces of the rings changes the uniformity of the mesh distribution, causing damage to the electromagnetic shielding efficiency.
With the increasingly complex electromagnetic environment, electromagnetic shielding optical windows are required to have increasingly high light transmittance and electromagnetic shielding capacity. Particularly in the fields of aerospace equipment and advanced optical instruments, optical windows has been required to have not only a light transmittance of 95% or higher, but also an extremely low impact on the imaging quality, and achieve a shielding efficiency of 30 dB or more in a microwave frequency range of less than 20 GHz, which is difficult to be realized by the existing technology. Both Patent 200810063988.0 and Patent 200810063987.6 utilizes a double-layer metal grid placed in parallel on both sides of the transparent substrate of the optical window, and the two layers of metal grids have the same unit shape and structure parameters. By optimizing the distance between two layers of grids, the electromagnetic shielding efficiency can be significantly improved without lowering the light transmittance. However, distribution of the high-order diffraction stray light of this double-layer grid structure is comparative to that of the single-layer grid structure having the same light transmittance, and cannot fully meet the requirements for low impact on the image quality in the fields of future aerospace equipment and advanced optical instruments.